1. Field of the Invention
This invention relates to an original-mount-moving-type image forming apparatus, that is, to an image forming apparatus, such as a copier, an image reading apparatus, an image display apparatus or the like, in which an original mount, on which an original is set, is subjected to a reciprocating moving drive (subscanning movement) relative to an original scanning unit (main-scanning unit), such as an original illuminating unit, an image reading unit or the like of the main body of the apparatus. Image formation, image reading or the like is performed during the process of forward movement or backward movement of the original.
The present invention also relates to a reciprocating driving device used, for example, for moving an original mount of a copier, or the like.
2. Description of the Related Art
As an example of a load performing reciprocating movement, there has been disclosed an original mount of a copier as shown in U.S. Pat. No. 4,660,957, a mirror mount or the like. For driving the original mount or the mirror mount, a reciprocating movement, composed of a constant-speed advance movement and a retreat movement, is synchronized with main-body driving for an image carrying member, a feed roller or the like. A mechanism has been used which uses a motor rotating in one direction to provide the reciprocating movement. That mechanism is also used as a driving source for driving the main body. Such a mechanism can comprise two spring clutches for transmitting and interrupting the driving force for advance and retreat movements, one or two solenoids for switching and a gear train are combined (see, for example, Japanese Patent Publication No. 63-43617 (1988)).
However, since spring clutches and solenoids have the following problems, a reciprocating driving device which does not use spring clutches and solenoids has been needed:
(1) A spring clutch has a large number of components. Hence, if variations exist in accuracy of respective components, phenomena such as noise, slippage, spring breakage, and combined rotation occur. Accordingly, it is difficult to control accuracy with respect to the various components.
(2) Since the volume and weight occupied by a spring clutch (a so-called iron mass) are large, the space occupied by the entire driving device becomes large particularly for a small copier. Thus, it is difficult to produce a small copier with an inherently relatively larger spring clutch. Furthermore, the incorporation of high-accuracy components is expensive, thus increasing the cost of such a copier.
(3) When the device is applied to the driving of an original mount of a copier or the like, the capability of providing a neutral state is needed in order to achieve proper synchronism for process conditions such as charging, transfer, fixing and the like, in addition to reciprocating movement.